The human sense of hearing is a sensitive system that relies on a series of overlapping queues that allow the listener to gain accurate insight to its environment as well as to identify the direction of sounds. Sound sources generate a set of acoustic cues, that is, sound sources have an acoustic signature, which the human sense of hearing analyzes differentially to localize these sound sources. Current art loudspeaker systems provide a high acoustic signature that the human sense of hearing will use to localize sound at the loudspeaker. This is a problem for audio and video sound systems that are designed to sonically “fool” the human hearing system into thinking that it is an environment other than where the listener actually is.
The human sense of hearing relies on three primary acoustic cues. The first acoustic cue is the interaural amplitude difference (“IID”), or the difference in amplitude. The amplitude or intensity of a sound will be greatest at the ear closest to the sound source. As a result sounds to the left will have a greater intensity in the left ear than the right ear and sounds coming from directly in front or back of a listener will be of equal intensity. These IIDs are most effective at frequencies nominally above 700 Hz where the acoustical wavelength is shorter than the human head. These amplitude differences are the result of the head casting an acoustic shadow over the far or opposite ear.
The second primary acoustic cue is the interaural phase or time difference (“ITD”), which is a result of the amount of time required for sound to reach each ear. The ear closest to the sound source receives the sound first and as a result the sound is perceived to be closer to that ear. Sound emanating from directly ahead arrives at the same time and will therefore be perceived to be directly ahead of the listener. The differential in arrival time predominates at frequencies nominally below 700 Hz where the acoustical wavelength is relatively long compared to the size of the listener's head. At frequencies below a nominal 150 Hz the sound becomes non-directional.
The third primary cue is the Head Related Impulse Response (HRIR) commonly expressed as the Head Related Transfer Function (HRTF). HRTF describes how a given sound wave input is filtered by the diffraction and reflection properties of the head, pima, and torso, before the sound reaches the eardrum and inner ear. Since each individual listener has a unique pinna, torso, and other physiological features that further modify the information presented to the sense of hearing. These listener-specific differences are referred to as the HRTF.
As sound increases in frequency its wavelength decreases. Sounds having a frequency above a nominal 700 Hz are increasingly blocked or shadowed by the head to the opposite ear while frequencies lower than a nominal 700 Hz are heard at equal intensity at both ears such that temporal or phase cues become dominant. As a result, the human sense of hearing relies on intensity cues for higher pitch sounds and temporal cues for lower pitch sounds, while HRTF cues are functions of essentially all audible frequencies with prior research indicating the range from 500 Hz to 4000 Hz to be most significant
FIG. 1 shows the spectral relationships of the three primary listening cues. The ITD cue 100 is dominant in the frequency range defined at the upper frequency where one wave length equals the width of the typical human head (nominally 2000 Hz) and at the lower frequencies where sound becomes non-directional (nominally 150 Hz). The IID cue 102 begins at about 700 Hz and becomes dominant through the upper frequencies to the threshold of hearing at 20,000 Hz. HRTF cues 104 influence localization principally from about 400 Hz and above.
The human sense of hearing uses the entire audible frequency range to localize sounds as detailed above. Additionally, the ITD cue determines direction when presented with conflicting directional cues; however direction is determined by intensity and HRTF cues when the low-frequency stimuli are removed. Thus, the sense of hearing is actually looking for the best match of the three primary cues, amplitude, phase, and HRTF, to accurately localize sound sources. In addition, the Precedence Effect or the Law of the First Wave Front states that similar sounds arriving from different locations are solely localized in the direction of the first sound arriving at the listener's ears, while later sounds, typically reflections and echoes, are ignored by the listener.
In addition to the primary localization cues the sense of hearing utilizes additional methods to improve localization in more difficult and potentially confusing environments. These techniques also complicate the reproduction of sound.
As explained above, when similar transient signals are perceived by the sense of hearing, the sense of hearing tends to localize by using only the leading edge of the transient. As an example, it is usually not difficult to localize a sound source in a reverberant room because the direct sound is received before the room reflections. The Law of the First Wave suggests that sound will reach a listener over a direct path followed by reflected sound; this is particularly true in a smaller room like those where a home audio and/or video system (“A/V system”) may operate. That sound arriving at the listener first creates the primary perception of direction.
The Precedence Effect describes how the sense of hearing will integrate reflections within nominally the first 50 milliseconds and combine them to give the impression that all the sound is coming from the initial source. This is similar to how the human sense of sight integrates the procession of still images into the sense we are watching a moving picture, provided that the still images are presented at a sufficient rate. In a typical listening room, loudspeakers are usually less than 12 feet from the listener(s), which equates to a transit time of about 10 milliseconds, which is shorter than the echo threshold. The Precedence Effect describes how the reflections that follow the first wave front are combined to give an increased sense of intensity.
As explained above, each individual listener has a unique HRTF. As a result, any single sonic event is perceived similarly by a number of listeners, but the actual information arriving at the inner ear will be different and is specific to each listener. In a prior art stereo system, the HRTF cues generated by the loudspeaker match the IID and ITD cues allowing the human sense of hearing to precisely locate the loudspeaker. This makes it difficult to fool the human sense of hearing into believing that the reproduced sound is not from loudspeakers.
FIG. 2 shows the temporal relationship between acoustic reflections and the first wave, or the direct sound, that initiates localization. When a sound is emitted from a source 200, propagation occurs in a spherical manner until the first wave 202 strikes a surface 204. This surface 204 causes the sound waves to be reflected as reflected waves 206. In a typical listening room, the portion of the first wave 202 that is moving toward a listener 208 will lead these reflected waves, causing the first wave 202 to initiate localization in the listener's 208 sense of hearing according to the Law of the First Wave/Precedence Effect. The arrival of reflected sound will always lag the First Wave Front of direct sound. In a sound reproduction system, early reflections of sound off room surfaces will serve as a sound level reinforcement as we know from the Precedence Effect.
An energy time curve (ETC) is a plot of amplitude in decibels against time and shows how sound energy decays in a space. Energy Time measurements are used in room and venue design to control acoustic reflections. Energy Time Curves can be utilized to better understand how the human sense of hearing uses time in the perception of an acoustic environment. How the time evolution of sound energy builds up in a room or venue is important since it is the energy in the sound that is critical for perception.
The generalized Energy Time Curve 300 shown in FIG. 3a illustrates the timing of direct and reflective sound waves. A sound generated at time t0 is propagated into the space reaching a microphone or listener at t1 on the first wave and is referred to as direct sound followed by early reflections shown at t2 with reverberation trailing at t3. In larger spaces, usually venues, reverberation is a factor, not so in small rooms where early reflections are typically shorter than 20 milliseconds. The length of time shown as ITD is the length of time between the direct sound and first reflections. The ITD gives listeners a sense of the characteristics of the space be it a cave, outdoors or a concert hall.
FIG. 3b is representative of an actual room measurement. This energy time curve 302 shows the level of reflections relative to the direct sound and the time it takes for sound to decay within the room. At time zero on the time axis, and the highest peak on the magnitude axis, represents the direct sound from the speaker. This plot shows how level changes over time, each peak is due to a reflection from a nearby boundary such as the floor, ceiling or side walls and clearly shows that there are a large number of reflections in a typical listening room and that sound does not always decay evenly.
The aim of stereophonic sound is to produce a three-dimensional illusion or a solid sonic illusion of the original sonic event. This ideal has been reduced by the IID, ITD, and HRTF cues generated by the right and left loudspeakers that localize the source of sound as coming from the loudspeaker. These cues are referred to as loudspeaker crosstalk and are an underlying reason for the dissatisfaction in the ability of prior art systems to generate stereophonic sound.
The prior stereo (i.e., two-channel) art requires two loudspeakers, left and right, driven by two independent electrical signals that represent the original sound. The right and left signals are the result of a recording process that ranges from a simple two microphone set up to elaborate multiple microphone arrays that are mixed together to create the final two channels. The left and right audio signals that are presented to each loudspeaker are composed of a combination of handed and center audio information. The result is that the handed information sounds as if it is coming from between center to either the left or right speaker and the centered information appears as a phantom image. The fact that both intensity and phase are equal for the centered information is the psychoacoustic mechanism that allows the phantom center image to be perceived. However, the listener must be on or very near to the centerline between the two speakers to perceive the center image. Furthermore, the recording microphones must have been coincident.
This arrangement, patented by Allen Blumlein in British Patent Specification No. 394325 and later referred to as “stereophonic sound,” improved upon single speaker monaural systems. However, “stereophonic sound” has its limitations. Serious frequency dependant comb filtering occurs due to the confluence of identical or high correlated center signals and, ideally, coincident microphones must be used to avoid recording phase differences. Additionally, utilizing recording techniques such as a middle/side shuffler circuit may be beneficial. Further, the stereo image will be located between, and not beyond, the loudspeakers and HRTF distortion will frequently occur because the loudspeaker HRTF cues rarely match those of the original venue. These requirements lead to localization smearing or collapse of the stereo effect. Current mult-channel or home theater systems can mitigate some of these problems but at great cost, complexity, and space requirements while exacerbating other problems.
The most serious problem with stereophonic sound is that it has proven to be very difficult to set up in the home due to the requirement that the “Stereo Triangle” be adhered to for the effect to be realized. The relationship between the positioning of the loudspeakers and the listener are critical. For correct reproduction the listener must be on the centerline between the speakers where each speaker is at a 30 degree angle to the listener as shown in FIG. 6a. While this angle has some flexibility the listener must be on the centerline in order for the “phantom center” image to be properly perceived. This is the key to the increased soundstage width that stereo is famous for. In most systems only one listener actually can hear this stereo effect since listeners to the right and left of the center listener will likely not perceive a stable center image and more likely will hear a right or left speaker predominate the soundstage. This is a real problem that has vexed the audiophile community for years. The additional speakers needed for multi-channel/home theater exacerbates the difficulty in set up.
Prior art loudspeakers have changed minimally since the advent of single-speaker monaural sound systems. The primary intent of the prior art loudspeaker is to energize the listening position directly. Prior art loudspeakers are built in several fundamental variations with the direct radiator or monopole the most common, while the omni-directional, dipolar, and bipolar types are less common. While certain designs aim to vary the direct/reflected sound ratio, they all share the common primary goal of directly energizing the space at the stereo listening position via direct sound from two locations right and left of the listener. Multi-channel or home theater systems add a center channel and two or more rear surround channels.
FIGS. 4a and 4b show typical Sound Pressure Level (“SPL”) charts of a direct radiator prior art loudspeaker. FIG. 4a shows the frequency-dependant radiation in the form of an SPL polar chart 400 of a prior art loudspeaker. The primary directional output lobe or on axis response 404 of a full range speaker assembly 402 is shown in polar form. Speaker assemblies typically utilize a single full range acoustical transducer or transducer array that uses multiple transducers each designed to reproduce a specific range of frequencies. The polar response changes from omnidirectional to directional as frequency increases, with the highest frequencies exhibiting a high level of directional behavior. Dipolar, bipolar and omni-directional radiators have fundamentally similar characteristics. The prior art loudspeaker is omni-directional below 2000 Hz nominally, while there is a significant loss of high-frequency information as data is taken off-axis. FIG. 4b describes the prior art direct radiator characteristics through various SPL vs. frequency charts 410. The frequency response demonstrates the characteristic reduction in higher frequency power as measurements are taken at 30° and 60° off axis. The design of a reduced signature loudspeaker requires techniques to address these characteristics.
FIG. 5 shows the effect when a sound source 200, like a loudspeaker, is located in a fixed position relative to the listener 208. Since the distance to each ear is constant the amplitude, timing and the HRTF cues generated all match, and therefore indicate, the location of the speaker. Recalling the energy/time curves of FIGS. 4a and 4b it should be noted that when the direct sound comes from a loudspeaker whose on axis directivity lobe is pointed at the listener, the listener will perceive the sound as coming from the loudspeaker location. As will be seen, a method will be disclosed to mitigate this localization.
In a stereo recording, the recorded HRTF cues are often different than that generated by the speaker location, causing confusion for the listener. In stereo recordings, amplitude is primarily manipulated to gain a sense of changing location or what we may refer to as realism. Although some phase or timing cues are recorded, the sense of hearing is looking for a match between all cues, which is not possible since the original recording and the location of the speaker are generating incongruent cues. As a result, the sense of realism and sonic accuracy suffers and confusion is increased.
Conventional stereo records one set of amplitude and phase cues representing the original performance; however, the use of the prior art loudspeaker results in the generation of a new set of amplitude, phase, and HRTF cues dependent on the location of the loudspeakers. This requires the sense of hearing to continually sort out these confusing localization cues resulting in listening fatigue and loss of clarity. The end result is that even the most accurate systems do not accurately reproduce the original event.
However, prior art loudspeakers identify the loudspeakers as the origin of sound because the three primary cues all match the loudspeaker location. The sensitivity of the sense of hearing is such that even a design that relies heavily on significant reflected energy may be readily identified. As a result, the recorded venue cues will be overridden and the sound is perceived to be reproduced in the listening room through the loudspeakers. Thus, a listener seated in the ideal position on the centerline of a soundstage will hear a sound stage composed of left at the left loudspeaker, right at the right loudspeaker and a phantom image is perceived at the center. The source of this problem is conventionally referred to as crosstalk, where each ear hears both loudspeakers.
Prior art loudspeakers create unnatural sonic situations because arrival time is critical to localization. For example, the creation of a phantom center image, that is, the appearance of a sound that is not generated by a sound source but a confluence of identical signals generated by two equidistant speakers and only if the listener is sitting on the centerline between these two speakers. In nature, sounds localized as coming from directly ahead originate from a single sound source that is directly ahead.
In addition, the fact that loudspeakers are physically static in a space generates another strong signature. FIG. 5 shows the temporal differential for different locations of sources in a quadrant of fixed radius. When a source is directly ahead the time differential between the ears is 0 milliseconds. The greatest time differential is for sounds directly to one side where the differential is related to the full diameter of the listeners head, nominally 0.70 milliseconds. When the sound source is in between these two extremes, the time differential may be approximately 0.35 milliseconds. As a result, a typical sound reproduction system will indicate to the human hearing sense that the “music” is coming from one location in the space, namely the location of the loudspeaker. This is in contrast to nature, for example in a concert hall, where sound comes from many localizable directions.
The current stereophonic system breaks down if the listener is not in the “stereo position.” The acoustic obviousness of the prior art loudspeaker requires that the listener be optimally located along the centerline between the two loudspeakers. As the listener moves away from center the sound shifts to the nearest loudspeaker and the phantom center image moves toward that side or, in some cases, disappears altogether. Multiple listeners must sit in the same line in order to gain a sense of the stereo image, which is neither comfortable nor practical. To reduce this problem, a third or center loudspeaker is required to deliver a combined left and right signal to better locate centered sounds for those sitting outside the “stereo position.” Multichannel sound accommodates multiple listeners more comfortably and now includes center and surround channels and loudspeakers. These additional channels assist in improving the perception of centered information for listeners away from the centerline, but they also complicate the number of sound sources that the sense of hearing must sort out.
Prior art loudspeakers are designed to directly energize the listening position(s) and not the listening space or room, which is a problem in the prior art. Additionally, the recording process must rely on higher frequency intensity cues to generate a wide sound field. This is effective in allowing good flexibility in the recording process but is another identifier in the localization chain.
The problems created by the conventional loudspeaker design and the recording process have resulted in separate groups of listeners, the stereo purist and the home theater listener. In either case, the effort required by the human sense of hearing to continually sort out the confusing soundstage results in listening fatigue and dissatisfaction.
The current understanding of loudspeaker crosstalk is that each loudspeaker communicates with both ears simultaneously, causing image blurring and other problems that lead to the loss of a true sense of a realistic sound stage. Current thinking is that the elimination of this crosstalk would allow true reproduction of recorded sound. However, the link between loudspeaker localization cues and crosstalk is not well controlled in the prior art.
Attempts to improve the stereophonic image generally focus on crosstalk reduction. A headphone that places a speaker over each ear is one solution and has proven dissatisfactory. The lack of HRTF cues is potentially a fundamental reason that headphones are disappointing. Headphones seem to be clearer and more intelligible, which appears to be a result of the lack of crosstalk. Amplitude crosstalk cancellation is often emphasized; however, phase and HRTF crosstalk are ignored.
Cross talk cancellation is achieved in the prior art through electronic and loudspeaker techniques.
Electronic cancellation techniques were first suggested by Atal and Schroeder in U.S. Pat. No. 3,236,949 directed to an “Apparent sound source translator,” and were later improved and commercialized by others. For example, U.S. Pat. No. 4,218,585 to Carver discloses an electronic device for canceling interaural crosstalk by inverting and modifying one stereo signal and recombining it with a modified version of the other stereo signal. Performing this operation on both stereo signals, Carver claims to effect a cancellation of interaural crosstalk when delivered by the loudspeakers. U.S. Pat. No. 4,308,423 to Cohen describes an electronic device for canceling interaural crosstalk and amplifying off-axis stereo images by using the difference of the right and left channels
These electronic cancellation techniques require very exact single seat set-up and may be upset by merely turning the listener's head. In addition, these systems appear to add new signals such that these systems prove unsatisfying; as a result none of these systems has found wide acceptance. The most fundamental flaw in crosstalk cancellation is that prior art loudspeakers generate phase and HRTF cues that complicate amplitude crosstalk reduction. Other electronic systems, like those introduced by Lexicon and the VMAx (virtual multi-axis) system developed by JBL/Harmon International have produced results that have not been well accepted in the market place.
Exaggeration techniques rely on accentuating certain cues to enhance the stereo image. These techniques increase the intensity of the L/R information by adding the difference signal in such a way that results in a 2R−L and 2L−R intensity exaggeration with the potential for some crosstalk cancellation. Others manipulate the spectral response using filters or digital signal processing (“DSP”) techniques to exaggerate the HRTF effects. Many of the HRTF techniques assume a one-size-fits-all philosophy by using a “typical” pinna, torso, and head function. It is difficult to program individualized HRTF parameters, so this is not provided. HRTF techniques generally prove dissatisfactory over the long term and result in inconsistent and unrealistic sound stages.
The techniques of exaggeration attempt to overpower the sense of hearing in a manner similar to that of intensity based two-channel stereo with similar results. New and extraneous signals are introduced into the listening space that further confuse and fatigue the listener.
Loudspeaker based cross talk reduction has been suggested in several patents, such as U.S. Pat. No. 4,058,675 to Kobayashi et al. This patent discloses a means for canceling interaural crosstalk using inverted and delayed versions of the left and right stereo signals fed to a second pair of speakers arranged to produce the correct geometry. U.S. Pat. No. 4,199,658 to Iwahara discloses using a second pair of speakers to reproduce the cancellation signal, which is composed of a frequency- and phase-compensated version of the inverted main signal. This cancellation signal is fed to a speaker just outside the main speaker on the opposite side from which the cancellation signal was derived. One of the few commercialized crosstalk reduction loudspeakers is detailed in U.S. Pat. No. 4,489,432 to Polk and uses a system similar to Iwahara but uses the stereo difference signal (L−R) for cancellation, amplifying the left and right components of the signal; this was not particularly well received.
The patent and prior art record shows several attempts to solve the problem of realistic sound reproduction by placing acoustic drivers at angles other than directly toward listeners but these have also not been successful or commercialized in even a limited sense. It is instructive to categorize these loudspeakers and detail their underlying weaknesses.
Reflective loudspeakers typically are found in two categories, the first aims certain acoustic drivers toward walls and/or ceilings intending that these will reflect off these surfaces with the hope of widening the sound field beyond the left and right speaker cabinet locations or improve realism. Examples are shown in U.S. Pat. No. 4,266,092 to Barker and U.S. Pat. No. 4,961,226 to Saffran as well as the Bose VideoWave TV video monitor and many others. The second reflective type uses conventional push-pull drivers that direct sound vertically and move air upward toward a reflector that converts the direction to horizontal. Examples are shown in U.S. Pat. No. 5,485,521 to Yagisawa et al., U.S. Pat. No. 5,446,792 to Sango and U.S. Pat. No. 5,615,176 to LaCarrubba that describe their Acoustic Lens Technology and used in Bang and Olufsen's BeoLab 5 speakers for example.
These reflective type loudspeakers are actually wide dispersion speaker systems that feature strong ITD cues from the bass drivers and frequently include midrange or high frequency acoustic driver(s) that are directed toward listeners that combine with enough HRTF and IID cues from the reflected sound for the sense of hearing to locate the loudspeakers. The sound has a different character, but the result is still unfavorable.
Other solutions try to solve the problems of the prior art by placing speaker drivers at extreme angles relative to each other. Some focus on single cabinet designs, while others focus on more traditional multiple cabinets as well as employing additional electronics in attempts to improve “spaciousness”.
One example is U.S. Pat. No. 5,553,147 to Pineau which places acoustic drivers in a single cabinet that is located on the center primary listening axis where the left and right drivers are 180 degrees apart (i.e., the left driver faces left and the right driver faces right). There may be some reduction of localization cues emitted by the loudspeaker at the cost of poor high-frequency response, but combined with very strong ITD and narrow HRTF cues; the listener's hearing sense will locate the array. While not addressed by Pineau it is likely that a severe high-frequency falloff will also be a deficit. The “back-to-back” type of speaker is a single cabinet omni-directional that may have limited merit for a solitary listener on the centerline, but proves unsatisfactory for listeners off the centerline that will readily localize sound at the loudspeaker.
U.S. Pat. No. 5,870,484 to Greenberger describes two systems designed to control ITD cues by controlling the directivity of the systems' low-frequency response. The directional radiation patterns have main radiation lobes pointing in different directions. An electronic system is described that combines an electronic processor with unique loudspeakers to produce a “Signal Dependant Radiation (SDR) gradient loudspeaker”. A loudspeaker only solution is claimed but not described that uses a wave-(guide) or horn loudspeaker solution. An objective in both systems is to increase the reflected-to-direct sound ratio. Greenberger focuses on the reduction of low-frequency (below a nominal 1500 hz) ITD. High-frequency IID cues as well as HRTF cues are neglected and the effectiveness of the ITD solution is not clearly shown leaving concerns that cabinet induced ITD cues and/or any leakage of the SDR solution will allow the sense of hearing to locate the loudspeaker. A number of transducer driver angles are proposed as well as many cabinet variations including the preferred embodiment, a coincident “back-to-back” type with similar problems to those detailed above with respect to Pineau's patent.
In this system an electronic solution uses modified stereo signals combined with unique loudspeaker configurations that are manipulated to produce a “signal dependant radiation gradient loudspeaker” with a polar response that directs more acoustic energy opposite the loudspeakers' location than toward center. The gradient-type loudspeaker requires unique signal processing electronics to be implemented, which is thus not compatible with prior art preamplifiers and processors. Alternately, additional boxes and controls may be required. The claimed loudspeaker solution requires the use of the wave-type (wave-guide) or horn loudspeakers to achieve a directional radiation pattern. These designs are not particularly popular due to size, cost, and sound quality problems especially in the low frequency spectrum acknowledged by Greenberger. Using low-frequency horns requires large cabinets that cause significant cabinet-induced ITD cues since cabinet cues will arrive before driver ITD cues and initiate localization. As a result no effort is made by Greenberger to detail the advantages of this loudspeaker solution.
Greenberger, Pineau, among others, show several single-cabinet designs that are intended to be placed in the center of the listening room. However, speakers must be spaced apart in order to generate HRTF angles of sufficient width to be believable. Narrow HRTF cues, as well as ITD cues, at minimum are responsible for the poor performance of coincident designs.
An effective but impractical solution is to place a large panel between the speakers and have the listener sit nose-to-panel, effectively splitting the room in acoustic two. This solution was suggested initially by Bock and Keele in 1986, and further developed by R. Glasgal; in a system he calls Ambiophonics. The technique produces results that are more consistent and realistic than electronic, loudspeaker, or combination techniques, but retain the solitary listener requirement.
These proposed solutions to the stereo problem demonstrate that there is a poor understanding of how to accurately reproduce sound. As detailed above, high acoustic signature sound generated by the loudspeakers essentially erases the original recorded venue cues. As will be seen, it is essential that, with the exception of a few specific cases, the listener must not be able to detect loudspeaker generated cues (i.e., IID, ITD and HRTF cues) on the first wave front that initiates perception as known and described by the Precedence Effect.
Prior art recording can range from simple two-channel right/left microphone recordings to elaborate multi-microphone arrays. Coincident and multi-microphone recordings tend to exaggerate directional cues. Recordings are typically made of several instruments and singers individually and then mixed into a pair of stereo signals. Various tracks are panned from center to left or right depending on the recording engineer's judgment. This is reasonable since stereo is fundamentally an amplitude driven medium and higher frequency sounds are more readily perceived as IID cues by the sense of hearing. The result may be unnatural, for example instruments may be spread across the sound stage such as a 20-foot wide drum kit with singers 20 feet apart. Prior art recording exaggerates left/right information and engineers tend to mix venues in the same recording which is common in current multi-track recordings. Multichannel recordings use similar techniques resulting in an increase in data storage space required by recordings.